The first one is a category error, in that cold infection is caused by a replication capable entity, the rhinovirus (the cold virus), while allergies tend to be largely caused by replication incapable entities (so-called “allergens“) such as excretory products of dust mites, cockroaches, cat sebaceous glands, etc. Thus, when considering immune responses against cold viruses versus those against “allergens”, there is a difference in kind in that viruses are subject to evolutionary selection pressures, namely anti-viral immunity, while this consideration is largely moot for “allergens”. This category difference not only mandates different prophylactic and therapeutic approaches for cold infection versus allergies but also implies that effective immune responses in these two instances are likely to be subject to different pressures.
The second error concerns the interpretation that allergies are products of an overactive immune system. While this may have been believed at one time, an abundance of subsequent research suggests that allergies are rather manifestations of dysregulation, not overactivation, of the immune system (1, 2, 3).
The connection between growing up on farms and incidence of cold infection appears to be indirect. On the one hand, early life exposure to farms correlates with lower incidence of asthma, hay fever and other allergies (4, 5, 6, 7, 8, 9, 10, 11, 12). On the other hand, cold infections tend to be more frequent, more severe and more long lasting in asthmatics compared to non-asthmatics (13, 14, 15).
It is well appreciated that persistent allergies have the capacity for extensive airway remodeling (16, 17). Airway remodeling refers to semi-permanent to permanent, long-lasting structural and functional changes in the upper respiratory tract that increase airflow obstruction and importantly for airway homeostasis, increased secretion of certain inflammatory mediators such as TGF-beta (18, 19). In turn, such changes render an allergic individual more susceptible to cold infections.
Thus, growing up on farms —> less likely to develop allergies —> less likelihood of airway remodeling and other immune dysregulation mediated tissue pathologies—> apparent lower susceptibility to cold infections.